It is widely known that a xenon oscillation could occur in the pressurized water reactor (PWR). The xenon oscillation is observed in the form of spatial oscillation of power distributions due to variations in spatial xenon concentration distribution. Xenon is a fission product from nuclear fission and has a high neutron absorption capability.
Xenon oscillation involves significant power distribution distortion in a nuclear reactor and may cause excessive local power, resulting in the local temperature rises. In order to ensure the safety of the core, xenon oscillation must be suppressed within a range of safe plant operation.
One of the technologies for preventing the above-mentioned excessive distortion of power distribution is known as “Power Distribution Control Method” for keeping axial power distribution within a prescribed range. For example, the Constant Axial Offset Control Method controls an axial offset (AO) within an allowable range which is determined based on the target-AO. Where, the AO is defined as a fraction of the power difference in the upper and lower halves of the core, and the target-AO is defined as the AO at which the core power is stable with control rods being almost totally withdrawn. In operation, it is monitored whether AO (or a value calculated by multiplying AO by the relative power when the core is at partial power) is within the allowable range, and if AO is out of the allowable range, the control rods can immediately guide AO back to the allowable range (Patent Document 1, Non-Patent Document 1).
In addition, this inventor also invented a xenon oscillation control method described in Japanese Patent No. 3202430 (Patent Document 2, Non-Patent Document 2, Non-Patent Document 3). The xenon oscillation control method employs an axial offset of the current power distribution (AOP) in a nuclear reactor core, an axial offset of the power distribution which would give the current xenon distribution under equilibrium condition (xenon-corresponding axial offset of the current power distribution: AOX) and an axial offset of the power distribution which would give the current iodine distribution under equilibrium condition (iodine-corresponding axial offset of the current power distribution: AOI). Xenon oscillation can be suppressed by inserting or withdrawing control rods, so as to guide a trajectory to plot a parameter DAOPX(=AOP−AOX) on X axis and a parameter DAOIX (=AOI−AOX) on Y axis, to the origin of the coordinates.